The present invention relates to epitaxial deposition and, in particular, relates to apparatus used in the deposition of the material.
Chemical vapor deposition (CVD) is one of the standard methods of manufacturing thin (epitaxial) crystalline layers. These layered structures are required in many semiconductor components including transistors and heterojunction lasers. During vapor deposition the reactants are introduced into a quartz tube reactor vessel, for example, in which a substrate is held by a substrate holder (susceptor). The susceptor is used to heat the substrate so that the reactants pyrolyze and deposit out onto the heated substrate. Typically, the reactants flow past the heated substrate at one end of the reaction vessel and are exhausted at a distant end. Heating is required for the vapor deposition crystal growth; since the walls of the reactor are relatively cool, the reactants should only pyrolyze at the heated substrate.
Problems that have arisen with conventional CVD reactors are a tendency towards non-uniform crystal growth due to variations in the reactant flow as it travels across the substrate and non-uniform heating of the substrate. Conventional MOCVD reactor flow channels attempt to obtain a well behaved, non-turbulent reactant flow by having a long entrance length and/or quartz inserts to shape the flow traveling across the substrate. Unfortunately, reactant gases can become depleted as they pass over the substrate and deposit unevenly, and also the perfection of the epitaxial layer depends upon several critical parameters and controlling the temperature of the substrate surface is one of these critical features. Advanced semiconductor materials are composed of two or more primary elements (i.e., GaAs, InP, CdTe, HgCdTe, GaInAlAs, etc.). Epitaxial deposition of binary, ternary and even quarternary layers are much more difficult than mono-elemental depositions normally encountered in silicon technology. These materials are much more sensitive to temperature variations. The layer thicknesses and electrical properties have been observed to vary by 100% across a two inch wafer that developed a mere 2 degree Centigrade per centimeter temperature gradient using conventional susceptors such as ones made of graphite.